This invention relates to a technology for enhancing controllability of output power based on a power control signal voltage of a high-frequency power amplifier circuit and gain""s linear characteristics of a gain control amplifier, and particularly to a communication semiconductor integrated circuit device having a high-frequency power amplifier circuit built therein, and a technology effected if applied to a wireless communication system such as a cellular phone or the like with the communication semiconductor integrated circuit device built therein.
A high-frequency power amplifier circuit has been built in an output unit provided on the transmitting side of a wireless communication system (mobile communication system) such as a mobile phone, a cellular phone or the like. The high-frequency power amplifier circuit has a configuration wherein a semiconductor amplifying element corresponding to a final stage comprises a discrete part (output power MOSFET or the like) and a gain control amplifier called a linear amplifier is connected to a previous stage thereof. The linear amplifier and a bias circuit for supplying a bias current therefor are formed on one semiconductor chip as semiconductor integrated circuits.
A system has been generally configured such that a cellular phone makes a phone call while changing an output (transmission power) so as to adapt to a surrounding environment according to a power level instruction signal sent from a base station in accordance with a use environment and avoids interference with other cellular phones. A high-frequency power amplifier module corresponding to an output stage on the transmitting side of a cellular phone of, for example, a North American 900 MHz-band standard system, a GSM (Global System for Mobile Communication) or the like has a configuration wherein a gate bias voltage of an output power element is controlled so as to reach output power necessary for calling according to a control voltage VAPC outputted from an APC (Automatic Power Control) circuit.
Meanwhile, a cellular phone of a conventional GSM or DCS (Digital Cellular System) is generally configured such that a gain control amplifier selects gain stepwise. In a cellular phone of a CDMA (Code Division Multiple Access) system on the other hand, a gain control amplifier, particularly, an amplifier for controlling the level of a transmit signal needs to linearly control gain. To this end, a linear gain converting circuit has been used which supplies such a bias current as to linearly change the gain of the amplifier with respect to an output control voltage VAPC supplied from a baseband circuit or the like, to the amplifier.
It is necessary that the bias current outputted from the linear gain converting circuit changes exponentially with respect to the control voltage VAPC. Thus, a linear gain converting circuit might be conventionally used which comprises a bipolar transistor Qa, a constant voltage source VBB for supplying a base potential to the bipolar transistor Qa according to a control voltage VAPC, and an attenuating amplifier ABB such as shown in FIG. 8 by way of example. The attenuating amplifier ABB is used because a variable range of the control voltage VAPC is about 0V to about 2V whereas a variable range of the base voltage of the transistor Qa is low as in a range of 0.7V to 0.9V.
While the bias circuit shown in FIG. 8 is configured such that the base voltage is controlled through the use of an exponential change in current flowing through a base-to-emitter PN junction of the transistor to thereby change an output current Iout exponentially with respect to the control voltage VAPC, the bias circuit is very sensitive to variations in base-to-emitter voltage VBE of the transistor Qa and variations in the gain of the attenuating amplifier ABB. It has therefore been found that a problem arises in that the bias current lout of the linear amplifier greatly changes due to variations in manufacture of each transistor and a variation in temperature, so that the relationship between the control voltage VAPC and the gain of the linear amplifier is not held constant and a gain variable range varies depending on products.
An object of the present invention is to enhance gain""s linear characteristics of a gain control amplifier employed in a communication semiconductor integrated circuit device equipped with a high-frequency power amplifier circuit including the gain control amplifier and a bias circuit for supplying such a bias current as to linearly change the gain of the gain control amplifier, and a wireless communication system using the communication semiconductor integrated circuit device.
Another object of the present invention is to provide a high-frequency power amplifier circuit which is excellent in controllability of output power based on a power control signal and high in power efficiency.
A further object of the present invention is to provide a wireless communication system and a high-frequency power amplifier circuit using the same, which is capable of prolonging a call time of the wireless communication system and the life of a battery employed therein.
The above, other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.
A summary of a typical one of the inventions disclosed in the present application will be described in brief as follows:
A bias current generating circuit, which supplies a bias current to a linear amplifier that constitutes a communication high-frequency power amplifier circuit, comprises a plurality of variable current sources respectively different in magnitudes of their current values and start level. These variable current sources are controlled according to an input control voltage to thereby combine their currents into a bias current. The combined bias current changes exponentially with respect to the input control voltage.
According to the above means, since the currents of the plurality of current sources are combined together without using voltage-current characteristics of a base-to-emitter PN junction of a transistor to thereby realize exponential characteristics of the bias current, a change in bias current due to a variation in base-to-emitter voltage of the transistor with both variations in base-to-emitter voltage of the transistor and a change in temperature with variations in manufacture can be avoided. Consequently, a bias circuit can be realized which supplies a stable and high-accuracy bias current to its corresponding linear amplifier.